Incorrect transects north of 65N

[aekiss]

All transect plots are incorrect where they extend into the tripolar cap north of 65N.

Two reasons for this:

1. transect paths following constant i (or constant xt_ocean) are not meridional in this region, due to curving of grid lines
2. vectors components (e.g. transports for MOC) are parallel and perpendicular to grid lines, so are not meridional and zonal in this region - eg see Bug at tripole seam access-om2#86

The error is probably smallish, since the coordinates don't twist around much in the Atlantic sector where such transects are normally done.

https://cosima-recipes.readthedocs.io/en/latest/Recipes/True_Zonal_Mean.html addresses 1. but not 2.

xESMF doesn't do vector regridding - you need to first rotate the vectors and then regrid the components - see https://xesmf.readthedocs.io/en/stable/limitations.html#vector-regridding

I don't know of a satisfactory conservative method for regriddding transport vectors, particularly if we want to maintain div and curl.

@navidcy just noting this here for good measure